A typical planar optical waveguide device includes a planar substrate, an array of waveguide cores supported on the planar substrate and a cladding layer. Optical radiation propagates in the cores. The lower index cladding layer confines the radiation to the higher index cores. In some cases, there is a second cladding layer between the cores and the planar substrate.
The planar optical waveguide device is designed to transport optical radiation across a two dimensional planar substrate surface. The device usually performs a passive function on the optical radiation so as to modify the output signal from the input signal in a particular way. Some examples of planar optical waveguide devices are as follows. Optical splitters divide the optical signal power in one waveguide into two or more waveguides. Couplers add the optical signal from two or more waveguides into a smaller number of output waveguides. Spectral filters, polarizers, and isolators may be incorporated into the waveguide design. WDM (Wavelength Division Multiplexing) structures separate an input optical signal into spectrally discrete output waveguides, usually by employing either phase array designs or gratings. A particular advantage of planar optical waveguide devices is the ability to include multiple functions on one platform. Active functionality can also be included in planar designs, where the input signal is altered by interaction with a second optical or electrical signal. Examples of active functions include switching (with electro-optic, thermo-optic or acousto-optic devices) and amplification.
In general, the key attributes for planar waveguide devices are optical loss, and process capability and cost. Process capability means the ability to write desired pattern of waveguide structures with good resolution and no flaws. Each device has its own specifications, which have to be met in addition to the more generic requirements.
To achieve planar optical waveguides, the current state of the art typically employs the following general process. First, a substrate is provided. The substrate is either silicon or silica, and is provided as a clean flat and smooth surface. In the case of a silicon substrate, a clad coating (a low index silica or silicate) is deposited. Next, a high index core layer (a silicate) is deposited on the substrate, with accurate thickness. The core and clad layer coatings are made from a flame hydrolysis technique, or a CVD technique or a plasma deposition technique. Next, the planar core layer is patterned to form an array of waveguide cores usually by some variation of a lithography/etch process. Finally, a low index clad layer is deposited to complete the waveguide structure. All of the variations on these process steps share an intrinsic high cost. Deposition times are long and patterning technologies are painstaking. The process is capable of forming high quality structures, with feature resolution of as low as 0.5 microns and low defect counts. In high value added applications such as WDM devices the process has shown some commercial feasibility. In other applications such as couplers however, costs are too high to compete with other technologies.
In view of the foregoing, it is an object of the invention to provide a planar optical waveguide device which overcomes the problems of the prior art. More specifically, it is an object of the invention to provide a planar optical waveguide device that is formed from a low-cost optical material with low absorbance, having a range of indices of refraction and that can be deposited rapidly with a majority of mass loss occurring in the non-solid state. It is also an object of the invention to provide a process for forming a planar waveguide device that obviates the need for lithographic techniques.